Abstract
The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O3) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity—WS and WW treatments, respectively) and O3 exposure (1.0, 1.2, and 1.4 times the ambient concentration—AA, 1.2AA, and 1.4AA, respectively) were carried out on Quercus robur L., Quercus ilex L., and Quercus pubescens Willd. seedlings, to study physiological traits (1. isotope signature [δ13C, δ18O and δ15N], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, Asat, and stomatal conductance, gs]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased Asat in Q. robur and Q. pubescens while water stress decreased it in all three oak species. Ozone did not affect δ13C, whereas δ18O was influenced by O3 especially in Q. robur. This may reflect a reduction of gs with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O3 on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by δ13C and by δ18O in all three oak species, while δ15N did not define plant response to stress conditions in any species. The δ13C signal was correlated to leaf water content (LWC) in Q. robur and Q. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O3 exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy.
Highlights
Oaks are widely distributed in the Mediterranean basin where drought and air pollution such as tropospheric ozone (O3 ) are the main limiting factors for productivity [1,2,3]
The seasonal course of Asat and gs differed significantly from June to September–October in each species, with higher values in early summer than in autumn in the deciduous oaks (Figure 1). δ13 C, δ18 O, and δ15 N were statistically different among species (Table 1), with higher δ13 C and δ18 O values in WS than well-watered plants (WW) plants of Q. robur and Q. pubescens (Figure 2). δ13 C and δ18 O increased in WS seedlings of all species relative to WW seedlings (Table 1, Figure 2). δ18 O was significantly affected by ozone and by water stress, while δ13 C was significantly affected only by water stress. δ15 N did not change among treated plants
In Q. pubescens, δ18 O was different between WW and WS in all ozone-level-treated plants, as well as between AA and 1.2AA and 1.4AA in WS plants (Table S1)
Summary
Oaks are widely distributed in the Mediterranean basin where drought and air pollution such as tropospheric ozone (O3 ) are the main limiting factors for productivity [1,2,3]. The diversity of oak species is the result of the adaptation to the environment [4]. Forests 2020, 11, 864 plants to adapt to chronic environmental stress through the change of morphological and physiological traits [5]. Reduced photosynthetic carbon assimilation is often reported as a result of stomatal closure under reduced water availability [9,10,11]. O3 impairs ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity and photosynthesis, inducing stomatal closure [12,13].
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
